Power line telephone system



Sept. 8, 1942.

R. K. HONAMAN ET AL POWERLINE TELEPHONE SYSTEM 3 Sheets-Sheet 1 Filed April 19, 1941 E h u 7 Qk O $33 EEQE K. HONAMAN T 056000 R /NVENTORS= .HI PERKINS By W-% E R. K. HONAMAN ET AL POWER LINE TELEPHONE SYSTEM Sept. 8, 1942.

Filed April '19, 1941 SS heetS-Sheet 3 U N ADM N MOI. 0. WWW f A K. I. max m 3 E v V W B A Q:

Patented Sept. 8, 1942 OFFICE POWER LINE TELEPHONE SYSTEM Richard K. Honaman, Glen Ridge, N. 1., and Dexter T. Osgood, Jackson Heights, and Kenneth H. Perkins, Valley Stream, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application April 19, 1941, Serial No. 389,284

11 Claims.

This invention relates to electric wave transmission systems, and, more particularly, to a power line carrier frequency telephone system.

Car ier frequency telephony on transmission lines primarily intended for the transmission and distribution of low frequency, high voltage electric power has, of .course, been known for some time, but has never been very extensively applied. The somewhat extensive electrification of rural areas that has taken place in the recent past and that appears to be projected for the fu ture, and the possibility provided by the rural tween a telephone station coupled to a power line and a central oflice or exchange for the telephone stations on the power lines.

Another object of the invention is to improve and to simplify the power supply to the power consuming relay, signaling and communication devices at the stations coupled to the power lines.

In accordance with this invention, a plurality of telephone stations and a central office or exchange for the telephone stations are coupled to a power line on which low frequency, high voltage electric power is being transmitted and distributed. Each telephone station comprises means for receiving from and transmitting to the power line a modulated high frequency or carrier wave. Each station may be arranged to transmit on one carrier wave and to receive on a second carrier wave, with means of interchanging the transmitting and receiving frequencies. The central office enables the connection of a telephone station on the power line to another station on the power line, and, also, to a party on another telephone system. Equipment for receiving and transmitting on the high frequency waves for the power line system may be incorporated in the central oflice, or, preferably, maybe included in difl'erent from'that for conversational purposes. the signal wave operating a signaling circuit at the telephone station that includes a gaseous discharge device whose output circuit includes relay means for connecting power supply to the high frequency equipment at the telephone station so that it will be energized when the called party answers, or so that the high frequency equipment will be energized for a preassigned .period only if the called party does not answer. Each telephone station is adapted to signal the central office on another high frequency or carrier wave, and to operate a gaseous discharge device signal and relay switching circuit at the terminal station to transmit a low frequency signaling current to the central office to operate a visual indicator of an incoming call thereto. All of the power required for operation of the relays and signaling and communication devices is preferably derived from the power line, and utilized either as the low frequency alternating current at transformed voltage or, after suitable rectifia high frequency or carrier terminal station enclosed in a suitable housing and supported on one of the poles carrying the power line. This terminal station enables communication and sigcation, as direct current.

A more complete understanding of the invention will be derived from the particular description that follows read with reference to the accompanying drawings illustrative of an embodiment of the invention wherein:

Fig. 1 is a schematic to show the general arrangement of a power line carrier frequency telephone system in accordance with the invention; Fig. 2 shows a preferred circuit arrangement at a telephone station coupled to the power line i of the system of Fig. 1;

Fig. 3 shows a preferred circuit arrangement for a carrier frequency terminal'set or equipment coupled to the power line of the system of Fig. 1, whereby telephone stations coupled to the power line may connect with a central office or ex-- Fig. 5 illustrates the natureof the carrier frequency circuit in the circuit arrangement of Fig.

Fig. 6 illustrates the nature of the carrier frequencycircuit at the telephone station shown in Fig. 1;

Fig. 7 shows a portion of the power line telephone system, with a subscriber station shown in schematic; and

Fig. 8 shows how the three sheets of these 'of the power line.

drawings should be arranged for greater ease in understanding this disclosure.

Descript on of the power line carrier frequency telephone system Fig. 1 shows in schematic a power line carrier frequency telephone system. The power line Ill is intended or designed for the transmission of electric power, for example, at high voltage and low frequency, and is adapted to have distribution transformers coupled to it for supplying power consumers along the line with electric power of low voltage, or lower voltage than that Such a. line might comprise a high voltage conductor l and a second con-. ductor lo, the latter being connected to ground at a multiplicity of points along its length and preferably at each point to which a distribution transformer may be coupled. Such a line may beof the pole supported type and be adapted to transmit alternating current of a commercial frequency, for example, 25 cycles per second or 60 cycles per second and of a potential of, for example, 7600 volts. Coupled to the power line at a plurality of geographically separated points are telephone subscriber stations 20, preferably at a location at which a distribution transformer is to be found. Each telephone station is adapted to be connected with the other stations and to communicate over the power line on an electric wave of frequency greater and preferably much higher than the frequency of the electric power being transmitted. One or more such carrier waves each of the order of several hundred kilocycles per second may be employed. Each telephone station, therefore, is provided with means to modulate high frequency electric or carrier waves in accordance with a telephone signal originating at the stations and to transmit such signals to the power line; and with.

means to demodulate a modulated high frequency electric or carrier wave incoming to the station, and to give a visual or aural indication of the modulation component. The modulated wave incoming to the station originates in one of the other telephone stations, or in a relay stationor terminal set 38 coupled to the power line and adapted to connect the telephone stations on the power line with a central oflice or exchange 40 whereby the subscribers on the power line telephone system may be connected with subscribers of another or a general telephone system. Each telephone station may be coupled to the power line through a distribution transformer, for example, as described in L. K.

signaling circuit 55. The circuit of the .telephone' station is coupled to the high voltage conductor ll) of the power line through a connection 45 and a suitably proportioned capacitor 56 of low impedance to the high frequency electric or carrier waves employed for telephonic purposes but of high impedance to the low frequency of the electric power being transmitted over the power .line. The capacitor 58 is connected to retard coll 302, which is connected to ground. The coil has a high impedance to the carrier waves and a low impedance to the lowqz' frequency of the electric power. The primary winding 51 of the distribution transformer 58' is connected at one end to the conductor I0, and at its other end to ground. One end of the secondary winding 59 may be connected to ground also, the other end being connected by conductor 60 to power consuming electric devices such as lighting fixtures, motors, etc., as well as being connected to the telephone station by conductoril. The potential developed across the winding 59 is preferably of some standard order, for example, 110 volts or 220 volts.

For purposes of this disclosure, it is assumed that, as has already been proposed in the art, each telephone station transmits on one high frequency electric or carrier wave Fl and receives on a second high frequency or electric or carrier wave F2 and includes means for interchanging the high frequency or carrier waves so that transmitting can be accomplished on wave F2 and receiving on wave Fl. The carrier set 50 is shown in greater detail in Fig. 6. The terminals 18 of the set are common to a transmitting circuit or path H, and a receiving circuit or path 12, being connected therewith through conductors 13. The transmitting circuit comprises the transmitter 53; an oscillator or other source H of the high frequency electric wave Fl or F2; a modulator a band-pass filter 11 adapted to transmit either or both side-bands of the modulated high frequency wave. The receiving circuit comprises the band-pass filter 18 adapted to transmit each or both of the side-bands of the incoming modulated high frequency wave; a demodulator I9 for translating the modulation component of the received modulated wave into voice frequency currents;

an amplifier with A. V. C. for amplifying the demodulator output to a preassigned degree; and the receiver 52 for translating the voice frequency current into sound waves corresponding to those produced at the calling station. The set also includes a suitable switching means, indicated by numeral 300, which changes the frequency of the oscillator 14 from Fl to F2 and interchanges the filters" and 18 when, for example, the button 311i is pressed. The amplifier, modulator,.demod'- ulato'r and oscillator may be of any known type,

for example, those employing electron discharge devices or vacuum tubes, the operating potentials and currents therefor being supplied through a suitable rectifier 8l, the input to which is derived from the transformer 58 through conductors 82, as will be described in greater detail hereinafter, Normally, that is, when the handset 5| is on its support 83, 'the telephone station carrier subscriber set 50 is'not energized and does not consume any power. The oscillator would be one adaptedto generate normally the high. frequency Fl, but. be provided with a manuallyoperated button 84, or other device, operation of which by the subscriber changes the constants of the oscillatorsoscillatory circuit, so that the electric wave generated by the oscillator is of a different high frequency F3 so long as the button remains operated, for a purpose explainedmore. fully hereinafter.

The station signaling circuit 55 comprises a pair of relays RIO, R20; an audible signaling means or ringer 90; a gaseous discharge device GTI a tuned high frequency circuit or filter 82; an antiresonant network 93; and a time-delay element or relay cut-out T.

The device GTI comprises an envelope con- I taining a filament or cathode, an anode and a control grid in a gaseous atmosphere, forex ample, of a rare gas, such as argon. This tube is of the type in which, for any given anode potential, there is a critical grid potential. It the grid is held more negative than this value and the gas is not ionized, the anode current will be zero; if the grid is made less negative, the anode current assumes a value determined by the anode potential and the impedance of the anode circuit. Once ionization or discharge has started, the grid loses control over the anode current. Potentials for anode and the grid and heating current for the filament are continuously supplied from the secondary winding 59 of the transformer 58, the primary winding 95 of transtube, the relay and ringer are energized by the pulsating current flow in the filament-anode circuit. The audible signal produced by the ringer notifies the subscriber at the telephone station of the incoming call thereto. An armature-andcontact set I01 is associated with the relay RI0.

- The armature is permanently connected to the former 95 being connected at one end to ground and at its other end, through conductors 6|, 91, to the ungrounded terminal of winding 59. The transformer 95 comprises a pair of secondary windings 98, 99, the former providing heating current for the filament, and the latter, grid biasing potential. A potentiometer I00 permits adjustment of the grid potential to take care of variations in tubes. The grid potential is preferably 180 degrees out of phase with the line voltage on the anode. The filter 92 comprises an inductance IOI and a variable condenser I02 connected in parallel in its series arm, and an inductance I03 in its shunt arm. This filter is tuned to a high frequency electric wave F4 other than those for signaling to the central ofiice or exchange, and for conversational purposes. The filter is connected to the power line conductor I0 through conductor I04 and the normally engaged springs 3, 4 of the assembly 54. In addition to discriminating against unwanted frequencies,-the filter increases the voltage of the signal incoming to it by a 'preassigned amount, and applies it to the grid of tube GTI through the grid leak and condenser network I05. v

The characteristic of the tube GTI should be such that there is nearly a linearv relation between the voltages on the grid and on the anode that just causes ionization and the establishment of anode current flow. Thus, with a grid potential just in excess of that required to produce ionization, the tube will be equally sensitive conductor BI, and its associated contact is permanently connected to one conductor of the power supply conductors 82 for the station carrier set, to spring 6 of assembly 54, and to the contact of the armature-and-contact set associated with relay R20.

The windingof relay R20 has one end connected to spring I of assembly 54, and its other end connected through a current-limiting resistor I09 to ground, to one ofthe station carrier set power supply conductors 82, and to the spring 2 of assembly 54. Connected in shunt of the winding of relay R20 is a thermally-sensitive resistance element T, of the type that decreases in resistance with increase in temperature resulting from current flow therethrough. The ele ment T is preferably proportioned so as to be initially of, high impedance, and of substantially no effect on current flow through relay R20; and to become of sufllciently low impedance in a preassigned time, for example, two or three minutes,

- that the relay is substantially. short-circuited.

for substantially the entire positive half of a C. P. S. electric wave. The high frequency output of the filter 92 is subtracted from the initial or normal grid bias, and is of such a value as to reduce the grid bias to a value at which the tube ionizes. During each positive half cycle of the power supply to the anode, anode current will flow, that is, so long as the high frequency wave input to filter 92 continues, there will be a pulsating current flow in the filament-anode circuit of the tube.

The network 93- is connected between the grid and the anode of the tube GTI to reduce the effect of the grid-to-anode capacity and of the I filament-to-anode capacity.

The armature of the armature-and-contact set I08 is permanently connected to the power supply conductor GI, and the contact isconnected to spring 6 of assembly 54.

The switching spring assembly 54 comprises the springs I, 2-4. When the handset 5I.is on the support 83, springs 4 and I are in contact with springs 3 and 6, respectively, and springs I, 2 and 5 are out of contact with support 83, spring 3 and spring 6, respectively. When the handset is removed from the support 83 ,the latter, to-

. gether with springs 3 and 6, moves upwardly under the influence of a suitable spring element (not shown) whereby the contacts between springs 3 and 4 and springs 6 and 1 are broken, and support 83, spring 3 and spring 6 make contact with springs I, 2 and 5, respectively.

The relay station or carrier terminal set 30 may be located at the central oflice or exchange 40, or, preferably, may be an isolated unit supported on one of the power line poles, suitably enclosed against the elements, and connected to the central ofiice 40 through a telephone pair I I0. The relay station comprises a high frequency electric or carrier wave transmitting and receiving set or equipment I20; a signaling switching circuit I30 comprising a two-stage discharge device circuit I40, and av plurality of relays RI, R2. R6; an oscillatorv I50 for generating a high frequency electric wave F4; and a hybrid coil I60 and balancing network N for coupling together the central oflice andthe circuits at the relay station.

' The carrier wave set I20; as shown in Fig. 5, comprises a transmitting branch or path I2I, and a receiving branchpr path I22 having common terminals I23, one of which is connected to ground and the other to the conductor I0 through the coupling condenser 56 and conductor I24. The receiving path comprises a band-pass filter I25 for transmitting one or both side-bands Whenever the high freof the modulated high frequency electric wave Fl; a demodulator I26 for translating the modulation component of the modulated wave into voice frequency currents; and an amplifier I21, preferably embodying automatic volume control, for amplifying the voice frequency currents, the amplifier output being supplied through the terminals I28 to the conductors I28, the latter being connected at their other ends to terminals 4, I on the section I60 of the hybrid coil I60. The transmitting path comprises a pair of terminals I3I to which are connected conductors I32. At their other ends, the conductors I32 are connected through the contacts 4, 6 of relay R2 to the terminals 4, 1 of the section I60" of hybrid coil I60. In the transmitting branch are a modulator I33, in which the voice frequency currents incoming to terminals I3I are modulated on the high frequency electric wave F2 originating in an oscillator I34; an amplifier I35 for amplifying the modulated wave F2; and the band-pass filter I36 for transmitting the amplified modulated wave to the terminals I23 and through them to the power line. As with the telephone station set 50, the amplifiers, modulator, demodulator and oscillator of the set I20 may be of. the type employing electron discharge devices, the operating potentials and currents therefor being supplied over conductors I31 from the power line through a distribution transformer 58 and a suitable rectifier I38, as explained more fully hereinafter.

The discharge device circuit I40 comprises an input network or filter I4I tuned to the high frequency electric wave F3, and discriminating against the other high frequency waves employed in the system. Its series arm is connected between the high frequency conductor-l24 and the grid leak and condenser network- I42, and its shunt arm is connected between ground and a point intermediate the series arm and the network I42. The electron discharge [device or vacuum tube VTI comprises a filament, anode and control grid. Its filament is continuously energized from the secondary winding I43 of transformer I44. The primary winding I45 of the latter has one terminal connected to ground and its other connected through conductor I46 to conductor I6l. The potentiometer I41 permits adjustment of the bias on the control grid. Anode potential is obtained from the high potential end of the primary winding of transformer I44 through connection I48, choke coil I48 and one winding of the interstage transformer II. The control grid is biased I80 degrees out of phase with the voltage applied to the anode. The tube VTI is coupled to a gaseous dischargedevice GT2,

- similar to the tube GTI, through a tuned circuit I is adapted to be supplied to relays R2, R3, R5.

as energizing current, through the connections I12, I13, I14.- The condenser I15 is an alternating current by-pass. Relay R4 is connected across a resistance I16 in the plate supply lead to the amplifier I of the terminal carrier set, I20. Relay R6 is connected to the terminals 4, 1 of hybrid coil section I60" through the contacts 4, 6 of relay R2, and is adapted to be operated by 20 C. P. S. alternating current originating at the central ofiice. relays'RI, R2 R6 will be described in detail in connection with a later description of the operation of the system disclosed.

The oscillator I50 is, for example, of the Hartley type and is adapted to generate a high frequency electric wave of frequency F4. The electron discharge device VT2 derives its anode potential from the power line through the distribution transformer 58 and transformer I11, the alternating current being supplied to the anode of tube VT2 only when relay R6 is energized and, therefore, its associated pair of contacts closed. The high frequency wave generated by the oscillator is applied to the power line through the coil I18 coupled to the coil I19 in the filamentanode circuit of tube VT2.

The central office or exchange 40 may be of I pair H0. The energizing winding of the operators signal I8I is connected in series with a blocking con-denser I81 across the line terminals I88 at the central ofllce, the drop being adapted to be actuated by an alternating current of 60 C. P. S. originating at the relay station. The circuit for the line windings of the repeating coil is normally interrupted by the open contact I associated with the jack I82, a normally closed contact 2 associated with the jack I82 being included in the circuit for the drop I8I.

Call from power line telephone station to the central ofllce The completion of a connection between a telephone station on the power line and the central ofiice will now be described, the connection orig- I inating at the subscriber's station A. Let it be assumed that the party at station A wishes to be connected with a telephone subscriber on another telephone system.

adjustment of the potentiometer I41, and its anode potential is derived through connection I55 from the high potential end of the winding I45.

Relay RI is connected in the filament-anode circuit of tube GT2 whose mode of operation is The party at station A removes the handset 5| from the support 83. Normally-engaged contact springs 3-4 and 6-1 of assembly 54 are disand 56; Engagement of spring I and support connects the carrier set 50 to the power line,

and engagement of springs 5-6 completes a.

the same as that of tube GTI. The rectifier I10,

power supplycircuit for the set 50 from the trans- .former 58 as follows: Ungrounded terminal of winding 58, conductors 6| and 20!, springs 5-6, conductors 82 to and from the rectifier 8I (Fig. 6), conductor 202 to ground return. Closure of springs 23 connects the input terminal of the The sequence of operation of the 2,294,906 network 02 to ground. The rectifier 3| supplies the oscillator, modulator, demodulator andam plifler with energizing current and potential. After a short wait for the thermionic devices in the carrier set to heat up, andafter listening to ascertain that the power line is not in use by another telephone subscriber on the line, the

*calling party operates the" button 84 to'cause the oscillator 'to generate the high frequency carrier wave F3. The high frequency wave F3 is impressed on the power line'through the ter- .minals 10 of the carrier set, and is transmitted the device GT2 continues, pulsating direct current flows inits cathode-anode circuit, and the relay RI is energized and its associated normally-open contact set is closed. Operation of relay R'I closes the following circuit for the ener-- gization of relay R2: One output terminal of rectifier I10, conductor I14, contact set of relay RI, winding of relay R2, conductor I13, back to the other output terminal of the rectifier I10. Energization of relay R2 causes its associated normally-open contact sets I, 2, 3, 5 to close, and normallyclosed contact sets 4, 6 to open. Closure of contact set 2 of relay R2 locks the latter in its operated condition over the following circuit: One output terminal of rectifier I10, conductor I14, normally-closed contact set 2 of relay R4, contact set 2 of relay R2, winding ofrelay R2, conductor I13 back to the other output terminal of rectifier I10. Normally the seriallyconnected outer windings of section I" of the hybrid coil are connected through contact sets 4, 8 of relay R2 to the voice frequency input conductors I32 of the carrier set I20. -Closure of contact sets 3, of relay R2 disconnects the hybrid coil winding, and, therefore, the telephone line I I0 from the conductors I32. Closure of contact set I' of relay R2 completes the following energizing circuit for relay R5: One output terminal of rectifier I10, conductor I14, contact set I of relay R2, winding of relay R5, conductor I12 to the other output terminal of rectifier I10. Normally-open contact sets I, 2 associated with relay R5 closewhen the latter is energized. Closure of contact set I of relay R5 locks the relay R5 in operated condition over the following circuit: One output terminal of I10, conductor I14, normally-closed front contact set cf relay R3, contact set I and winding of relay R5, conductor I12 to the other output terminal of rectifier I10. Closure of contact set 2 of relay R5 completes the alternating current power supply circuit to the carrier set current power supply circuit tot he carrier set I 20 and to the oscillator I50. The power supply circuit for the carrier set maybe traced as follows: From the non-grounded terminal of the secondary winding of transformer 58, conductor I6I, conductors I31 to and from the rectifier I38 in the .carrierset I20, conductor 203, contact set 2 of relay R5, conductors 204 and 205 to ground return. The power supply circuit for the oscillator may be traced as follows: From the non-grounded terminal of the secondary winding of transformer 58, conductors IBI and 206, primary winding 201 of transformer I11, conductors 208 and 203, contact set 2 of relay R5, conductors 204 and 205m ground*.retuifn.

The thermionic devices in the amplifiers, modu-' lator, demodulator and oscillator of the set I20 are energized from the output of the rectifier I38; and the'tube VT2 from the filament-and anode windings of transformer I11. As the tubes of the amplifier I35 heat up, the potential drop across the resistance I16 will reach a preas- I signed value and cause relay R4 to operate, to

close its associated normallyopen contact setv I and to open its normally-closed contact set 2. Closure of contact set I of relay R4 closes the following circuit: Non-grounded terminal of the secondary winding of transformer 58, conductor I6I, contact set 5 of relayR2, serially-connected upper windings of hybrid coil section I60, contact set 3 of relay R2, contact set I of relay R4, conductors 204 and 205 to ground return. The alternating current induced in the hybrid coil line winding 209 operates the magneto drop I8I at the central ofiice, to provide the operator with a visual indication. The'opening of contact set 2 of relay R4 breaks the holding circuit for relay R2 and the latter releases. Therelease of relay R2' restores its associated contact sets I 6 to the condition shown on the drawings, but relay R2 is sufficiently slow in rel asing to permit the alternating current signal to the cen, tral ofiice to operate the drop I8I. Relay R5 does not deenergize as a result of the restoration to normal of contact set I of relay R2, but remains operated through the holding circuit including its own contact set I.

The operator responds to the signal by inserting the plug I83 into the jack I82. Normallyclosed contact set 2 of the jack is opened to break the circuit for the signal I8I, and normally-open contact set I of the jack is closed to complete a direct current circuit through the normally-open line winding ofrepeating coil I 80. This circuit is as follows: One output terminal of rectifier I10, conductors I14 and 2I0, hybrid coil winding 209, one conductor of line IIO, the repeating coil line winding including the jack contact set I, the other conductor of line IIO, hybrid coil winding 2, winding of relay R3, conductor I13 to the other output terminal of rectifier I10. Relay R3 operates. The opening of its front contact set breaks the holding circuit for relay R5, and the latter releases; but closure of the back contact set of relay R3 in parallel with the contact set 2 of relay R5 maintains the alternating current supply to the carrier set I20.

When the operator. answers the signal from the subscriber, i. e., speaks to the subscriber, the audio frequency currents originating at the central ofiice are transmitted over the telephone line IIO through the hybrid coil I60, contact sets 4, 6 of relay R2 and conductors I32 to the'set I20. In the modulator I 33 they are modulated onto the carrier wave F2 from the oscillator I34. The modulated carrier wave is amplified in amplifier I35 and transmitted through the filter I36 and power line terminals I23 to the power line. At the-subscribers station, the modulated carrier' wave enters the carrier set 50 throughterminals 10, is transmitted by filter 18 to the demodulator 19, and the demodulated output is amplified" by, the amplifier 8'0 and transmitted to the receiver, 52. The subscriber at station A havingrestoredfi the button or key84, the output of the oscillato 84 will be at its normal frequency, that is, of,car rier wave FI. When the calling party talks the operator, the sound waves aretranslate 'b the transmitter 53 into voice frequency currents which are impressed on the modulator 15, in which they are modulated onto the carrier wave. Fl supplied by the oscillator 14. The modulated carrier wave is amplified by amplifier 18 and transmitted through the filter 11 to the terminals 10, and are impressed on the power line. At the relay station, the modulated carrier wa've enters the terminal set I20 through terminals I23, and. is transmitted by filter I25 to the demodulator I28, the, audio frequency output of which is amplified in amplifier I21 and transmitted through terminals I28 to the conductors I29. The conductors I29 are connected to the outer ends of the serially-connected lower windings of hybrid coil section I60. The audio frequency currents will be induced in winding 2H and the winding connected to the balancing network N. The currents induced in winding 2 are transmitted over the line H and through the repeating coil,

the jack and the plug to the operators set. If it is assumed that the party at station A wishes to be connected to a party on another telephone system, the operator completes the connection through the conventional switchboard and central oflice cord facilities (not shown). When the call is finished, the subscriber at station A operates the button 84 to transmit the signal wave F3 to the power line and to the relay station. This wave is amplified by the tube VTI and impressed on the tube GT2, the pulsating output of tube GT2 operating relay RI and relay R2 as already described. Relay R2, however, does not ing I19 are induced in winding I18, and impressed on the power line.

Each telephone station will be in the condition shown for station A (except, of course, the station on the power line at which the call originated, if'such is the case), and the high frequency wave F4 will be impressed on the input network 92 of each station through connection 45, contact springs 3-4 of assembly 54 and conductor I04. The network 92 selectively transmits the signal wave F4 to the control grid of the gas tube GTI. The normal grid potential is reduced sufficiently to permit ionization during each half cycle that the anode potential is positive, whereby pulsating direct current fiows in the cathodeanode circuit of the tube so long as the signal wave F4 is incoming to the station. Ringer 90 and relay RIO operate, the former to give an audible indication of the incoming call, and the latter to close its associated contact set I01. Closure of contact set I01 completes a power supply circuit for the carrier set 50 over the following path: Non-grounded terminal of winding 59 of transformer 58, conductor 8|, contact set I01, conductors 82 to and from the rectifier III of the set 50,' conductor 202 to ground return.

lock up since its holding circuit is held. open at 4 contact set 2 of relay R4. During the periodthat relay R2 is operated, its contact sets .3, 5 are closed, and contact sets 4, 8 are opened. The

serially-connected upper windings of hybrid coil section I60", therefore, are connected during that period to the alternating current supply, and the alternating current is induced in hybrid coil winding 209 and transmitted to the central omce over the line I I0 to operate a supervisory relay or magnetic drop in the cord circuit thereat. When the' operator disconnects the cord circuit, the direct urrent circuit holding relay R3 operated is bro en at contact set I of the Jack I82. Relay R3 releases, disconnecting the alternating current power supply from the carrier set I20, causing relay R4 to release, and restoring the relay station to its non-operated condition.

Call from central oflice to power line telephone station Let it now be assumed that the operator at the central office wishes to signal a telephone station, for example station A, to complete a call from another subscriber on the power line or from a party in another telephone system.

The operator inserts plug I83 in jack I82 completing the direct current circuit for relay R3 through contact set I of the jack. Closure of the back contact set of relay R3 connects the alternating current power supply to the set I20 and the oscillator I50. After a short interval in which the filament of the tube VT2 may heat up, the operator operates key I84 to connect low frequency signaling current from source I85 to the repeating coil. Relay R6 is tuned by means of the series condenser 2I2 to operate on this signaling current, which reaches it over line IIO,

the hybrid coil, contact sets 4, 8 of relay R2, and conductors 2I3. -Closure of the front contact set of relay R5 completes the anode potential circuit of the tube VT2. The oscillations of high.

frequency or carrier wave F4 developed in wind- The relay R20 operates over the following circuit: Non-grounded terminal of winding 59 of transformer 58, conductor 6|, contact set I01, conductor 2I4,- contact springs 81.of assembly 54, winding of relay R20, resistance I09, conductor 202 to ground return. Energization of relay R20 causes contact set I08 to close, to hold relay R20 in operated condition over the following circuit: Non-grounded terminal of transformer winding 59, conductor GI, contact set I08, contact springs 8-1 of assembly 54. winding of relay R20, resistance I08, conductor 202, to ground return. alternative power supply circuit forthe carrier set 50, namely: Non-grounded terminal oftransformer winding 59, conductor 5|, contact set I08, conductors 82 to and from rectifier 8|, conductor 202 to ground return. Relay R20 remains operated until the handset 5I is removed from the support 83, thereby opening its energizing circuit at the contact springs 6-1 of assembly 54; or, if the handset 5I is not removed from its support, relay R20 remains operated until the temper'ature sensitive variable resistance T decreases in resistance sufilciently to substantially shortcircuit the relay R20. Prior to either event, however, the output of the rectifier 8| energizes the thermionic devices in the set 50. The incoming signal equipment at each station will be operated when the signal wave F4 is impressed on the power line, but by use of a suitable code, the particular station desired by the operator will be informed thereof. When the called subscriber removes his handset from its support, the holding circuit for relay R20 is broken at contact springs 61 of assembly 54, the carrier set is connected to the power line by engagement of contact spring I and support 83 of assembly 54, power supply to the rectifier 8I of the set '50 is insured by engagement of contact springs 58 of assembly 54, and engagement of contact springs 2--3 of assembly 54 connects the ungrounded terminal of network 92 to ground. When the called party Closure of contact set I08 provides an r l handset on its support to restore the station circuit to the condition shown in Fig. 2.

Connection between two telephone stations on the power line The manner in which a connection would be established between two stations A and B coua signaling carrier wave, a gaseous discharge device including a cathode, an anode, and a control electrode, means to apply audio frequency alternating current to said cathode and anode and to bias said control electrode to a potential maintaining said device normally deionized, means connecting said network and said control electrode for applying said signaling carrier wave to said electrode to overcome the bias thereon party at station A operates button 30! on his set 50 after informing the operator that he desires connection with station B on the same power line. When the latter answers, conversation between the parties at stations A and B takes place over the power line. Station A.transmits on fre-v quency F2 while station E receives on frequency F2, and station A receives on frequency Fl while 'station B transmits on the latter frequency. Operation of button 30| by the party at station A interchanges the normal transmitting and receiving frequencies for station A. The operator's plug remains in the jack I82 so that relay R3 remains energized and the operator is thus informed that the power line is in use. When the conversation has been concluded, the calling party, here the party at station A, signals to the operator on the carrier wave F3 before replacing the handset on its support. Withdrawal of the plug I83 from the jack I82 restores the relay station to its non-operated condition.

Although this invention has been disclosed with reference to a particular embodiment, it will be understood that it is not limited thereto but only by the prior art and the appended claims.

What is claimed is:

1. In a power line carrier frequency telephone system comprising a plurality of carrier frequency telephone stations connected to said power line, means at each station comprising a network selective to a signaling carrier wave, a gas-filled tube including a cathode, an anode,

and a control electrode, means to apply audio frequency alternating current to said cathode and anode and to bias said control electrode to a potential maintaining said tube normally deionized, means connecting said network and said control electrode for applying said signaling carrier wave to said electrode to overcome the bias thereon whereby said tube ionizes during the half cycles of said alternating current in which said anode is at positive potential and generates an audio frequency pulsating current, and means connected to the cathode and anode of said tube to be actuated by said pulsating'current.

2. In a power line carrier frequency telephone system, a power line for transmitting audio frequency alternating current electric power and a carrier frequency telephone station connected to said power line, said station comprising carrier wave equipment including circuits for transmitting to and receiving from the power line the carrier wave modulated in accordance with audio frequency currents corresponding to sound waves originating at the telephone station or at an-- other station connected to the power line, means at the station comprising a network selective to whereby saiddevice ionizes during the half cycles of said alternating current in which said anode is at positive potential and generates a pulsating audio frequency signal, and means connected to said anode and cathode to be actuated by said pulsating signal.

3. A power line carrier frequency telephone system comprising a power line for transmitting 'low frequency, high voltage alternating current electric power, a plurality of telephone stations coupled to said line and adapted to receive and to transmit modulated carrier waves over said power line, means at each station selective to a signaling carrier wave of frequency other than that used for modulated carrier wave transmission, a gaseous discharge device comprising a cathode, an anode and a control grid, energized with alternating current derived from said power line, said device normally being deionized because of excessive grid bias, means connected to said selective means to apply said signaling carrier wave to said grid to overcome the grid bias sufficiently to enable the anode to take control during the positive half cycles of the alternating current, so that the device develops a pulsating direct current in its cathode-anode circuit, and means in said cathode-anode circuit to be actuated by said pulsating direct current.

4. In a power line carrier frequency telephone system, a power line and atelephone station coupled to said power line, said station comprising a carrier wave equipment including circuits for transmitting to and receiving from. the power line a carrier wave modulated in accordance with audio frequency currents corresponding to soundwaves originating at the telephone station or at another station connected with the power line, means for connecting and disconnecting said circuits from the power line, said circuits being normally disconnected from said power line, anormally-ope'n power supply connection between said power line and said carrier'- wave equipment-means responsive to a signaling carrier wave of frequency other than that of the audio frequency modulated carrier wave for completing said power supply connection,

and means to restore said power supply connection to its normally-open condition after a preassigned time. has elapsed if the carrier wave circuits of said carrier wave equipment are not connected to said power line by the party at the telephone station.

5. A power line carrier frequency telephone system comprising a power line, a plurality of telephone stations coupled to said line' and adapted to receive and to transmit over said power line carrier waves modulated in accord means to supply an audio frequency alternating current to said device, an input circuit for said device connected to said selective means whereby is applied directly to said device to causeit to ionize on alternate half cycles of said audio frequency alternating current and to deliver an audio frequency pulsating output, an output circuit for said device, and means in theoutput circuit of said device to be actuated by said pulsating output.

circuits for said carrier set, a relay in the outputcircuit of said device to be operated whensaid device ionizes and to close one of said power supply circuits, a second relay to be operated when said first relay operates and to close a second of said puWel' supply circuits, the operation of said relays providing successively different power supply circuits for the carrier set and the operation of said second relay providing the latter with an energizing circuit independent of said first relay whereby said second relay re- 4 mains operated after said first relay ceases to be operated, said second relay including an energizing winding, and means in shunt with the winding of said second relay to deenergize it after a preassigned interval if the called subscriber does' not respond.

'7. A telephone system as claimed in the preceding claim in which said shunt means comprises a thermosensitive resistance element having a negative temperature coeflicient of resistance.

8. In a power line carrier frequency telephone system, a power line for transmitting low frequency electric'power, a plurality of telephone stations connected to said power line and adapted to receive from and to transmit to said power line a carrier wave modulated in accordance with audio frequency currents corresponding to sound waves, a central ofiice for said telephone stations connected to said power line,.

and means for signaling a telephone station from said central .ofiice, said means comprising a thermionic oscillator to be operated directly by alternating current from said power line and adapted to generate a high frequency signal wave, said oscillator having a normally-open power supply. connection between it and the power line, and a normally-open connection between its output circuit and the power line for impressing the signal wave on the power line, means succarrier wave transmitted by said selective means 7 cessively to close said power supply and signal wave connections whereby the oscillator generates the signal wave and the latter is impressed on the power line, and means at each station selectively responsive to said signal wave to indicate that said signal'wave is incoming thereto. 9. In a power line carrier frequency telephone system, a power line for transmitting low frequency electric power, a plurality'of telephone stations connected to said power line and adapted to receive from and transmit to said power line a carrier wave modulated in accordance with audio frequency currents corresponding to sound' waves, a central office for said telephone stations connected to said power line, and means for signaling a telephone station from said central office comprising a thermionic oscillator to be operated directly on alternating current from. said power line, for generating a high frequency signal wave, a normally-open power supply connection between said power line and said oscillator, means for closing said power supply connection to said oscillator, a normallyopen connection between the output circuit of said oscillator and the power line, means for closing said connection between the oscillator output and said power line, means at said central oflice for successively operating said two last-mentioned means whereby said power supply connection to the oscillator and the oscillator output connection to the power line are closed, and means at each telephone station selectively responsive to said signal wave to give an indication at the station that said signal wave is incoming thereto;

10. In a power line carrier frequency telephone system, a power line for transmitting low frequency electric power, a plurality of carrier frequency telephone stations and a carrier terminal station for said telephone stations connected to said power line, and a central ofllce for said telephone stations connected to said carrier terminal station, means at each telephone station for impressing a signal wave on said power line, means at said carrier terminal station responsive to said signal wave incoming thereto from said power line to transmit low frequency alternating current derived from said power line to said central oflice, and means at said central office operating on said alternating current to indicate to the operator at the central office that a call is incoming thereto.

11. The power line carrier frequency telephone system as claimed in claim 4 in which said restoring means includes a resistance element that undergoes a substantial change in impedance with change in its temperature in response to power supplied to it from said power line.

RICHARD K. HONA MAN. DEXTER T. OSGOOD. KENNETH H. PERICINS. 

